The present invention relates to electronic ballasts for powering high-intensity and a high-pressure discharge (HID) lamps such as high-pressure mercury and metal halide lamps, and to lamp fixtures using the same.
More particularly, the present invention pertains to electronic ballasts for HID lamps with circuitry for controlling power provided to the lamp.
FIG. 12 is a schematic of a conventional electronic ballast. The details of the circuit configuration and operation thereof are described later referring to FIG. 1 and the following is merely a summary thereof. In the ballast of FIG. 12, when the output power provided to an HID lamp DL is determined, the load voltage to the lamp DL through is detected by a load voltage detecting circuit 21 as part of a ballast control circuit 2, and is used to provide a prescribed load current according to the monitored value of the load voltage. For example, data according to voltage-current (V-I) characteristic table as shown in FIG. 10 may be stored in a memory 22 in the control circuit 2 so that a load current I is provided according to the detected value of the load voltage V. When determining the load current in this configuration, if the load current values for positive and negative polarities are different in a rectangular wave, this may result in flickering of the discharge lamp DL. Therefore, the detection period of the load voltage is fixed in one of the positive and negative half cycles, so that the load current to be provided in one cycle thereafter is determined according to the detected value of the load voltage.
FIG. 13 conceptually shows waveforms according to conventional detection (reading) of load voltage and a reflection thereof on control of the load current. The detection of the load voltage is fixed in a prescribed period of a positive or negative polarity. In the example shown in FIG. 13, the load voltage detection (reading) operation is fixed in a positive half cycle and the load current to be provided in one cycle thereafter is determined according to the detected value of the load voltage.
In a case where the load voltage is detected in a half cycle of a positive or negative polarity to thereby determine the load current to be provided in one cycle thereafter, as in the conventional example shown in FIG. 13, the ballast is normally operated when the load voltage values of both positive and negative polarities are generally equal. However, when the load voltage values of positive and negative polarities are largely different (for example, in a half-wave discharge or lamp rectification state), the target values of the load currents are different in the positive and negative polarity periods, and therefore a deviation is inevitably caused with respect to a target value of the load current in any one of the polarities. This is due to whether the polarity of the load current to be determined is the same as, or inverse to, the polarity of the monitored or measured load voltage.
As shown in FIG. 14, for example, in the case where the load voltage is detected in a polarity period of lower voltage, the load current in a polarity period thereafter is equal to or greater than a suited value, resulting in that the output power obtained (multiplying load voltage and load current) exceeds a prescribed value. Accordingly, there is concern for failure of the ballast or reduction in life of the lamp.
FIG. 4 shows waveforms of load voltage and load current when power being equal to or greater than prescribed value is actually provided by a ballast for powering a 150 W HID lamp. In FIG. 4, Vla denotes load voltage and Ila denotes load current. In this example, the load voltage is always detected in a positive polarity period and the target value of the load current is determined in the next both positive and negative polarity periods. Thus, the output power is generally equal to the prescribed value in the positive polarity period but is equal to or greater than the prescribed value in the negative polarity period.
In a configuration as disclosed Japanese Patent Publication No. 2815230), a circuit is adapted to disable operation of the ballast when the load current is largely different from a prescribed target value due to lamp rectification. However, in the case of an HID lamp, lamp rectification (a half-wave discharge state) is usually caused at the beginning of a lamp starting operation. In addition, after the lamp is started, it may take several minutes or longer to restart the lamp due to an increase of a gas pressure inside the lamp.